In a typical coherent Radar Cross Section (RCS) measurement, the magnitude and phase of the target return is collected over a range of frequencies. The frequency source is swept linearly over a band of frequencies, producing a linear-FM or "chirp" waveform. If high quality RCS data is required, there are two parameters of the chirp waveform (sweep) that must be closely controlled: linearity and frequency accuracy. Linearity is a function of each individual chirp. It describes the variation in the rate of change of frequency during the chirp. Ideally, the chirp will be perfectly linear, that is, a plot of frequency versus time will result in a straight line. A very linear chirp is required if the data is to be processed by a Fast Fourier Transform (FFT) algorithm (i.e. imaging). Processing non-linear data will result in a loss of resolution and focus. Frequency accuracy is a measure of sweep-to-sweep variations in the chirp waveform. Ideally, each chirp will be identical, that is, the oscillator will tune through the same frequencies each sweep. Frequency accuracy is very important if data from multiple sweeps is to be compared (i.e. radar cross section vs. angle plots).
For applications in the microwave frequencies, the YIG-Tuned Oscillator (YTO) is the most common frequency source. The YTO has many desirable features. It is small, inexpensive, broadband, fairly linear, has very repeatable sweep-to-sweep tuning characteristics, high output power, can change frequency quickly, and can be easily and accurately tuned with an analog voltage.
In order to provide a higher degree of linearity and frequency accuracy, the YTO is usually placed in a Phase Locked Loop (PLL) circuit to form a frequency synthesizer. A synthesizer can produce very accurate and repeatable frequencies. However, a finite amount of time is needed for the synthesizer to attain phase-lock. This time can often be in the range of 10-30 milliseconds. This drastically slows the rate at which data can be taken. For example, a fully phase-locked sweep of 128 points over a 6-18 Ghz bandwidth could take as much as 5 seconds to complete. Without the PLL, the YTO could tune over this bandwidth in as little as 20 milliseconds, about 250 times faster. One approach to improving the sweep speed involves only phase locking to the first frequency in the sweep. A synthesizer sweeping in this fashion could probably produce the above 6-18 Ghz sweep in about 50 milliseconds. This a sizable improvement, but, because there is no phase lock, linearity and frequency accuracy are sacrificed, the very reasons the synthesizer was used in the first place. Sweeptime is of interest because of the large amount of data required for accurate radar cross-section measurement and imaging.
In order to achieve faster sweep speeds and lower the cost, complexity, and size of the frequency source, the YTO is run "open loop", not phase-locked. This allows the radar to sweep a 12 Ghz bandwidth in about 20 milliseconds. Faster speeds are possible. Although YTO's are fairly linear devices they are not as linear as necessary for accurate and repeatable radar cross-section measurements.